Re: [PATCH 1/2] softirq: fix integer overflow in function show_stat()

[Leizhen (ThunderTown)]

On 2023/7/25 10:00, Leizhen (ThunderTown) wrote:
> 
> 
> On 2023/7/24 21:50, Matthew Wilcox wrote:
>> On Mon, Jul 24, 2023 at 09:22:23PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
>>> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>>>
>>> The statistics function of softirq is supported by commit aa0ce5bbc2db
>>> ("softirq: introduce statistics for softirq") in 2009. At that time,
>>> 64-bit processors should not have many cores and would not face
>>> significant count overflow problems. Now it's common for a processor to
>>> have hundreds of cores. Assume that there are 100 cores and 10
>>> TIMER_SOFTIRQ are generated per second, then the 32-bit sum will be
>>> overflowed after 50 days.
>>
>> 50 days is long enough to take a snapshot.  You should always be using
>> difference between, not absolute values, and understand that they can
>> wrap.  We only tend to change the size of a counter when it can wrap
>> sufficiently quickly that we might miss a wrap (eg tens of seconds).

Sometimes it can take a long time to view it again. For example, it is
possible to run a complete business test for hours or even days, and
then calculate the average.

> 
> Yes, I think patch 2/2 can be dropped. I reduced the number of soft
> interrupts generated in one second, and actually 100+ or 1000 is normal.
> But I think patch 1/2 is necessary. The sum of the output scattered values
> does not match the output sum. To solve this problem, we only need to
> adjust the type of a local variable.

However, it is important to consider that when the local variable is changed
to u64, the output string becomes longer. It is not clear if the user-mode
program parses it only by u32.

> 
> 
>>
>> .
>>
> 

-- 
Regards,
  Zhen Lei